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1994 LS400 Bad Performance/Rumbling/"Shaky" Idle

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This is kind of hard to explain without you feeling it, but basically the car shakes or rumbles when driving or sitting. You could also call it a shudder.

This happens at pretty much all speeds, but is less noticeable when going faster.

The RPM's don't change much, if at all when this happens.

It is very intermittent, but it has become a problem because I will try to floor the car to overcome this and all of the sudden the car will full send.

It also makes the car stall, again being very dangerous.


I have ran the car out of fuel about a dozen times now because the fuel gauge/full dash lights don't work.


The car will always start just fine which makes me think it is more serious than a bad fuel pump/clogged filter.


There are no engine lights on (they should work, they come on when the key is at acc).

The "Trac" orange light will come on sometimes (not to be confused with the "Trac Off" light that does not work) , but doesn't seem to correlate with the issue.


Apologies for the bad description, but it is really the best I can come up with.

Thanks for any help you can offer.

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Running the car out of fuel will wear out the pump whether intentional or not. The gas helps to keep the pump cool and when there's no gas the pump can overheat. The car is 25 years old and if it's on the original pump it might be due for a change. Changing the pump is not that hard. you just have to pop up the bottom rear seat cushion by pulling up hard on each end and it snaps out. Then take off the bottom bolts to the seat back. there are also hidden bolts behind the headrests that are covered by flaps then you have access to the gas tank. the pump itself is not that expensive.

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  • 1 month later...

I removed successfully the seat back easily without touching the rear seat bottom. Use a 1/4 ratchet with a 12 inch extension and you should be fine. 

To help reduce the fuel pump work, I recommend to change the fuel filter. 

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